Code division multiple access (CDMA) is a form of multiplexing and a method of multiple access that divides up a radio channel not by time (as in time division multiple access), nor by frequency (as in frequency-division multiple access), but instead by using different pseudo-random code sequences for each user. CDMA is a form of “spread-spectrum” signaling, since the modulated coded signal has a much higher bandwidth than the data being communicated.
In cellular telecommunications, the term handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another. In telecommunications there may be different reasons why a handoff might be conducted. For example, the handset might be moving away from the area covered by one base station and entering the area covered by another base station. In such a case, the call might be transferred to the second base station in order to avoid call termination when the handset gets outside the range of the first base station.
Multi-technology handsets can also perform a handoff, but the handoff can be between different technologies. This type of handoff becomes difficult, however, in a handset that uses CDMA as one of the technologies because the registration process in CDMA is very closely coupled with the powering on and off of a CDMA radio in the handset and acquiring a CDMA network.
A CDMA only handset performs registration (e.g., CDMA power-up registration) on acquiring an acceptable CDMA network and overhead settings. The CDMA only handset will continue to perform periodic registrations (e.g., timer-based, location-based) based on the settings indicated by CDMA overhead messages. The CDMA only handset will perform de-registration (e.g., CDMA power-down registration) when the CDMA radio is turned off. This process works adequately on CDMA only handsets.
In a multi-technology handset where CDMA is one of the technologies, the introduction of a new technology becomes a handicap because of the close coupling between the CDMA registration process and the powering on and off of a CDMA radio. For example, it would be highly undesirable and user-unfriendly to have to power off the handset in order to perform a handoff away from CDMA to a second technology. A handset that supports multiple technologies should be able to seamlessly handoff from CDMA to another technology and vice-versa but if the handset must be powered on and off each time this happens, such a scheme becomes impractical.